1. Field of the Invention
This invention relates to toughened silicon-ceramic composite bodies, especially those produced by a reactive infiltration process, e.g., reaction-bonded bodies. Reaction-bonded silicon carbide having a boron carbide filler or reinforcement particularly exemplifies the invention.
2. Discussion of Related Art
Reaction bonded silicon carbide (SiC) ceramics combine the advantageous properties of high performance traditional ceramics, with the cost effectiveness of net shape processing. These materials provide high surface hardness, very high specific stiffness, high thermal conductivity, and very low coefficient of thermal expansion (CTE). The processing consists of two steps. First, a carbon containing near net shape porous preform is fabricated; and second, the preform is reactively infiltrated with molten Si to form a primarily SiC body.
Reaction bonded silicon carbide ceramic offers extremely high levels of mechanical and thermal stability. It possesses low density (similar to Al alloys) and very high stiffness (˜70% greater than steel). These properties lead to components that show little deflection under load, allow small distances to be precisely controlled with fast machine motion, and do not possess unwanted low frequency resonant vibrations. In addition, due to the high stiffness and hardness of the material, components can be ground and lapped to meet stringent flatness requirements. Moreover, as a result of very low coefficient of thermal expansion (CTE) and high thermal conductivity, reaction bonded SiC components show little motion with temperature changes, and are resistant to distortion if localized heating occurs. Also, due to an excellent CTE match with Si, reaction bonded SiC ceramics are well suited as substrates for Si handling operations. Furthermore, both Si and SiC possess refractory properties, which yields a composite with good performance in many high temperature and thermal shock applications. Finally, dense, high purity SiC coatings can be applied when extremely high purity and/or superior resistance to corrosion are required.
Reaction bonded SiC ceramics have many outstanding properties, including high specific stiffness, low coefficient of thermal expansion, and high thermal conductivity. However, they typically also exhibit low fracture toughness, and are therefore not suited for many applications where impact loading will occur.
Thus, materials investigators have experimented with various techniques for enhancing the toughness or impact resistance of such inherently brittle ceramic-rich materials. Perhaps the most popular approach has been to incorporate fibrous reinforcements and attempt to achieve crack deflection or fiber debonding and pull-out mechanisms during the crack propagation process.
More recently, some have alloyed the brittle silicon phase with different metals such as aluminum, to enhance toughness. For example, the assignee's aluminum-toughened SiC provides a nominally 75% increase in fracture toughness relative to its standard reaction bonded SiC. This toughness allows the composite to be used in applications where some impact will occur. In addition, the composite can be used in thin walled component designs that would be difficult to produce with a low toughness ceramic.
The presence of the aluminum results in an increase in thermal conductivity relative to the standard SiC product, which is valuable in heat sink applications or in components where localized heating can occur. In addition, the thermal conductivity is in excess of that of most metal matrix composites because no additional metallic alloying elements are used, e.g., magnesium.
Silicon is usually thought of as being a brittle material, but this statement pertains to its ambient temperature characteristics. A review of the literature finds that Si undergoes a brittle to ductile transition in the 500° C. temperature range. (J. Samueles, S. G. Roberts, and P. B. Hirsch, “The Brittle-to-Ductile Transition in Silicon,” Materials Science and Engineering, A105/106 (1988), pp. 39–36.) Warren observed that this temperature can be influenced according to the density of dislocations in the silicon. Specifically, he observed that the transition temperature decreased when dislocations were introduced to a silicon surface by a grinding operation. (P. D. Warren, “The Brittle-Ductile Transition in Silicon: The Influence of Pre-Existing Dislocation Arrangements,” Scripta Met., 23 (1989), pp. 637–42.)
Moreover, Gogotsi et al. of Drexel have achieved metal-like ductile machining of silicon at room temperature by controlling the stress state at the cutting tool-to-workpiece interface. (Y. Gogotsi, C. Baek, and F. Kirscht, “Raman microspectroscopy study of processing-induced phase transformations and residual stress state in silicon”, Semicond. Sci. Tech. 14, (1999), pp. 936–44). Analysis of the Silicon crystal structure directly under the cutting tool by Raman microspectroscopy shows that the room-temperature ductile machining is obtained through a pressure-induced transformation of Silicon from the cubic diamond phase into a metallic beta-tin structure. This latter phase has mechanical properties of a typical metal and deforms plastically under the tool. Thus, under the correct loading conditions, ductile behavior of Silicon at room temperature can be obtained. Phase transformations in Silicon, as well as in boron carbide, can lead to additional energy dissipation, which is important in many applications, possibly including armor applications.
U.S. Pat. No. 3,857,744 to Moss discloses a method for manufacturing composite articles comprising boron carbide. Specifically, a compact comprising a uniform mixture of boron carbide particulate and a temporary binder is cold pressed. Moss states that the size of the boron carbide particulate is not critical; that any size ranging from 600 grit to 120 grit may be used. The compact is heated to a temperature in the range of about 1450° C. to about 1550° C., where it is infiltrated by molten silicon. The binder is removed in the early stages of the heating operation. The silicon impregnated boron carbide body may then be bonded to an organic resin backing material to produce an armor plate.
U.S. Pat. No. 3,859,399 to Bailey discloses infiltrating a compact comprising titanium diboride and boron carbide with molten silicon at a temperature of about 1475° C. The compact further comprises a temporary binder that, optionally, is carbonizable. Although the titanium diboride remains substantially unaffected, the molten silicon reacts with at least some of the boron carbide to produce some silicon carbide in situ. The flexural strength of the resulting composite body was relatively modest at about 140 MPa. A variety of applications is disclosed, including personnel, vehicular and aircraft armor.
U.S. Pat. No. 3,796,564 to Taylor et al., filed in 1967, discloses a hard, dense carbide composite ceramic material particularly intended as ceramic armor. Granular boron carbide is mixed with a binder, shaped as a preform, and rigidized. Then the preform is thermally processed in an inert atmosphere with a controlled amount of molten silicon in a temperature range of about 1500° C. to about 2200° C., whereupon the molten silicon infiltrates the preform and reacts with some of the boron carbide. The formed body comprises boron carbide, silicon carbide and silicon. Taylor et al. state that such composite bodies may be quite suitable as armor for protection against low caliber, low velocity projectiles, even if they lack the optimum properties required for protection against high caliber, high velocity projectiles. Although they desire a certain amount of reaction of the boron carbide phase, they also recognize that excessive reaction often causes cracking of the body, and they accordingly recognize that excessive processing temperatures and excessively fine-grained boron carbide is harmful in this regard. At the same time, they also realize that excessively large-sized grains reduce strength and degrade ballistic performance.
More recently, the assignee has likewise developed a reaction bonded boron carbide composite material because this system was thought to show potential as a candidate armor material, even if previous efforts suffered from various shortcomings. As examples of this “potential”, hardness is believed to be important in making an armor material having high mass efficiency. Moreover, many armor applications such as aircraft and body armor, require low mass. Boron carbide possesses both of these characteristics. None of the individual components of this reaction bonded boron carbide system possesses inherent toughness.
U.S. Pat. No. 3,725,015 to Weaver discloses a process for making low porosity, essentially defect free, composite refractory shapes via a reactive infiltration process. A carbon-containing preform is infiltrated with a molten metal containing at least two constituents. One of the constituents is capable of reacting with the carbon to form a metal carbide in situ in the preform. The other constituent is added such that the infiltrant alloy has a thermal expansion close to that of the refractory material making up the matrix of the preformed shape, thereby regulating, and preferably eliminating residual stress and microcracking upon cooling to ambient temperature form the processing temperatures. Weaver furthermore discloses providing to the infiltrant alloy a metal corresponding to the metal of the refractory material for the purpose of preventing the infiltrant from leaching out of the metal constituent of the refractory material. For example, the incorporation of about 6 percent by volume of boron in silicon saturates the alloy sufficiently to prevent its dissolving boron out of a boron carbide refractory material matrix.
3. Discussion of Commonly Owned Patent Applications
International Patent Application No. PCT/US99/16449, filed on Jul. 23, 1999, and which published as Publication No. WO 01/07377 on Feb. 1, 2001, teaches that reaction-bonded or reaction-formed silicon carbide bodies may be formed using an infiltrant comprising silicon plus at least one metal, e.g., aluminum. Modifying the silicon phase in this way permits tailoring of the physical properties of the resulting composite, and other important processing phenomena result: Such silicon carbide composite materials are of interest in the precision equipment, robotics, tooling, armor, electronic packaging and thermal management, and semiconductor fabrication industries, among others. Specific articles of manufacture contemplated include semiconductor wafer handling devices, vacuum chucks, electrostatic chucks, air bearing housings or support frames, electronic packages and substrates, machine tool bridges and bases, mirror substrates, mirror stages and flat panel display setters. The contents of this commonly owned patent application are expressly incorporated herein in their entirety by reference.